This invention relates to facsimile transfer over digital networks, and, more particularly, to the transfer of facsimiles over digital networks in real-time, dealing with delays caused by the networks.
Various protocols exist for transmission and receipt of facsimiles over low-delay analog voice-grade telephone lines. One such protocol is Group 3 (G3), defined in T.30 (xe2x80x9cRecommendation T30 (07/96)xe2x80x94Procedures for document facsimile transmission in the general switched telephone networkxe2x80x9d), which is hereby incorporated herein by reference. T.30 is an International Telecommunication Union (ITU) recommendation dealing with facsimile transfer over a telephone network. This recommendation defines three different protocols for facsimile transfer known as Group 1 (GI), Group 2 (G2), Group 3 (G3). Only the Group 3 protocol is in common use today.
While the G3 protocol is defined for analog transmission, the backbone network for modern telephone systems is generally digital. Accordingly, G3 facsimile is often carried over a digital telephone network using recommendations specified in ITU G.766, which is hereby incorporated herein by reference. The ITU G.766 recommendation describes how analog signals are demodulated, forwarded over the network and remodulated at the remote end of the call.
In addition to being used with the PSTN (Public Switched Telephone Network), G3 facsimile is also used over satellite and other digital networks. Even in non-PSTN networks, the techniques described in G.766 can generally be used, although they must be modified appropriately to suit the characteristics of the network.
The Protocol Analysis (PA) technique described in G.766 is widely used in non-PSTN networks. This technique requires that the Facsimile Interface Unit (FIU, i.e., the hardware/software component of a network that provides an interface between a compatible facsimile machine and a network) follow the G3 facsimile protocol so it can use the correct voiceband modems to demodulate signals from and remodulate signals to the G3 facsimile machine.
Various problems exist when trying to use G3 facsimile over digital networks, the main problem being the additional delay added to the connection by the nature of the digital network. For example, if a satellite link is used in the digital network, this adds approximately 250 ms of delay in both directions of signal travel.
A call through a digital network between two widely separated geographic locations may pass through several network components that introduce additional delay as compared to a direct analog connection between two facsimile machines. The delay problem becomes more acute when G3 facsimile is sent over a packet-switched network. Typically, these networks have no upper bound on delay. Further, the delay between any two packets is not predictable due to various network events. Unlike a modern PSTN, data passing through a packet network experiences variable and unpredictable delays.
The G3 facsimile protocol was designed for low-delay analog telephone connections and it contains no explicit ways to tell a facsimile machine to wait longer than some time period for protocol interactions to occur. The G3 facsimile protocol specifies several timers which may expire (timeout) if the delay is high. The most-used timer is one used for commands and responses. When an FIU transmits a command, it starts the timer and waits for a response message. If the timer expires, it retransmits the command and starts the timer over again. As soon as the facsimile machine starts receiving a response, it stops the timer. The facsimile machine ends the call after three retransmissions without reply.
In a high-delay situation, the retransmission of the command by the facsimile machine will often occur around the same time that the response to the original command is arriving over the network, resulting in a collision between the second copy of the command (being transmitted by the facsimile machine) and the reply to the first copy of the command arriving over the network. This situation is shown in FIG. 1 wherein timeout occurs without the originating facsimile machine (FTE) receiving a reply from the destination FTE 10-2.The retransmission from the originating FTE 10-1 collides with the reply from the destination FTE 10-2. The response of a facsimile machine to a protocol collision is vendor-specific and not predictable, and often results in failure to transfer a document.
ITU recommendation G.766 specifies one technique for handling delay situations and this is that the FIU will start to send so-called xe2x80x9cfill dataxe2x80x9d which is formatted to look like the normal preamble signal before a valid message. This fill data is sent to the FTE if a reply to a command has not arrived after several seconds but before a protocol timeout (triggering a retransmission) will occur at the FTE. FIG. 2 shows an example of the use of fill data. As shown in FIG. 2, the originating FTE 10-1 starts a timer at the end of its transmission. Then, before the timeout period expires, the FIU 14-1 starts sending preamble fill signals to the originating FTE 10-1 in anticipation of receiving a message over the network. When the FTE 10-1 starts receiving the fill data, it stops the protocol timer currently in progress. The fill data is sent by the FIU 10-1 in anticipation of receiving a reply to the original command over the network.
This delay compensation technique does not work well in several situations, including, for example:
The response message can be delayed so long that the total length of FIU-generated fill plus the response message exceeds the limits of the G3 facsimile protocol and the FTE thus rejects the message.
The response message does not arrive over the network at all. This could happen for many reasons including that the message was lost in the network or that the original command was corrupted during remodulation to the remote FTE.
The FTE violates the G3 facsimile protocol specification by failing to wait the full, specified timeout period before starting retransmission.
Another problem is variable delay over the network during page data transfer. G3 facsimile uses synchronous voice-band modems and, once transmission starts, a steady supply of page data is required for successful transmission. If the arrival of page data over a network is delayed unpredictably, this will create gaps in the remodulated data stream which will produce corruptions in the printed page image. One solution to this problem could be to increase the amount of buffering at the destination FIU 14-2 so that it can send data from the buffer during periods where no network data is available. There are, however, two problems with this solution:
1. The amount of buffering required is equal to the sum of all instantaneous delays throughout the call. Since these delays are unpredictable, it is impossible to establish an upper limit to the amount of buffering required for any call.
2. Additional buffering of this kind increases delay.
It is desirable to provide a facsimile system with the following features:
The system can perform document transfer between G3 facsimile equipment over digital networks of unlimited, variable and unpredictable delay.
The system has adaptability on a per-call basis to network connections of different capacity.
The system has interoperability with a broad spectrum of G3 facsimile equipment currently in use.
The system requires no changes to normal PSTN operator procedures for sending G3 facsimile documents.
The system provides little or no disruption to the appearance of the printed document.
Accordingly, this invention provides G3 facsimile through digital networks of high or unpredictable delay. The techniques of this invention are insensitive to delay and still have a high degree of compatibility with the diverse population of G3 facsimile equipment currently in use. There is little or no alteration of the facsimile document as a result of using this invention.
In one general aspect, this invention is a protocol for transferring G3 facsimile over a digital network of unlimited delay. The protocol compensates for an unlimited and unpredictable amount of delay through a digital network.